THIS INVENTION relates to an RF (radio frequency) transponder identification system and protocol and more particularly to a system and protocol to enable an interrogator to identify a transponder from a group of transponders.
Techniques have been devised over the years whereby a number of remote stations share a common channel of communication. From the early days of the simple Aloha technique, to the circuit-switched telephone networks, techniques are evolving and include the present day network medium access control techniques such as token ring (IEEE 802.5), token bus (IEEE 802.4) and Ethernet or CSMA/CD (IEEE 802.3) standards.
However, these medium access techniques are not suitable for a system such as RF identification whereby the remote station""s processing capability is largely limited to inexpensive transponders having a low-clock rate and low processing power. Furthermore, the transponders do not have a defined network structure which therefore prohibits the use of xe2x80x98tokenxe2x80x99 passing techniques.
Due to the nature of the radio frequency field, the transponders also do not have the capability to detect the presence of other transponders in a similar way to carrier detection schemes such as in CSMA/CD.
A number of multiple access techniques for systems such as Radio Frequency Identification have been developed and are discussed below.
In European Patent Application 0,689,151 (see FIG. 1 of the accompanying drawings), a system is described whereby a transponder in the field transmits a request-to-transmit signal (RTT) after counting a randomly generated number of time periods and awaits a first acknowledgment from the interrogating terminal. Upon receiving the first acknowledgment, the channel is assumed free and the transponder proceeds to transmit its identification information. A second acknowledgment is sent by the interrogating terminal after verifying the received identification information. The disadvantage with this system lies in that there is a need for two transmissions, one to request the use of the channel and another to perform the actual transmission of the identification information. This process introduces unnecessary time and data communication overheads and slows the system as a whole. Another disadvantage lies with the use of multiples of fixed time-slots between requests-to-transmit. In such a system the use of fixed time-slots results in unnecessary delays when no transponder is transmitting in that time slot. It can be assumed that the length of the second transmission is much longer than that of the request-to- transmit signal. As such, fixing the time-slots too far apart would result in unnecessary waits whilst the channel is free whereas fixing the time-slots too close to one another would result in more collisions.
In another technique described in U.S. Pat. No. 5,523,749 (See FIG. 2 of the accompanying drawings), the transponders are made to send repeated copies of its identification information. Such a system has the merit of simplicity and is aptly suited for simple systems where the number of transponders in an electromagnetic field is controlled. However, in applications where the number of transponders is large or the same identification information is carried by two or more transponders, then the system would not be able to read all the transponders or would take an indefinite amount of time to read all the transponders. This is because the repeated transmission by the transponders increases the probability of collisions by a factor equivalent to the number of repeated transmissions.
In yet another technique described in European Patent Application 0,685,825 (see FIG. 3 of the accompanying drawings), the system incorporates an interruption to the interrogation signal to indicate that the identification information has been successfully read. The transponder has means to sense the interruption and to cease transmission in response to the interruption xe2x80x98gapxe2x80x99. This improves the performance of the system as compared with the system described in U.S. Pat. No. 5,523,749. It should, however, be noted that the technique is inherently flawed in that given a situation where two transponders at differing distances from an interrogator are transmitting their identification information at the same time, it is possible that the interrogator is able to correctly receive the information from one of the two transponders, if the received power from the second transponder is much lower than the received power from the first, such that the contribution of the second transponder is insignificant so as not to cause any interference to the received identification information from the first transponder. The interrogator would continue to interrupt its interrogation signal indicating correct reception. This interruption, being received by both the transponders would cause both transponders to cease transmitting their identification information, resulting in the xe2x80x98lossxe2x80x99 of the second transponder. On a less probable but not impossible note, it is common knowledge that environmental noise can also cause occasional glitches in the interrogation signal and if not properly dealt with, this could be mistreated as an interruption indicator.
In the technique described in European Patent Application 0,685,825, the duration or length of each slot is taken to be a multiple of the time taken to transmit one identification sequence. Similar to that described in European Patent Application 0,689,151, this fixed time-slot does not reflect the state of the channel and causes undue delays and is thus not optimized.
In yet a further technique described in U.S. Pat. No. 5,550,547, a tree-splitting algorithm is used whereby all transponders in the electromagnetic field begin transmission. If more than one transponder is present, the interrogator fails to read and sends a xe2x80x98Failxe2x80x99 command. Upon receiving the xe2x80x98Failxe2x80x99 command, the transponders generate a binary random number. Transponders which generate a zero increase a state counter by one and stop transmitting. Transponders which generate a one keep their state counter at zero and attempt transmission again. Only transponders with a state counter of zero may transmit. All transponders with a state count not equal to zero will increase their state counter whenever a xe2x80x98Failxe2x80x99 command is sent by the interrogator thus keeping them further from transmitting. This process is a tedious one and would take a long time when the number of transponders is large. Furthermore, a collision is inevitable whenever there is more than one tag in the field. However, the algorithm guarantees that all transponders can be identified, given enough time. In applications whereby the interrogator or the transponders are mobile, time is a critical parameter and the system would not be suitable.
The present invention seeks to provide an RF transponder identification protocol which does not suffer from the disadvantages or problems associated with the above-described techniques.
Accordingly, one aspect of the present invention provides an RF transponder identification system comprising: an interrogator operable to transmit a plurality of marker signals; a plurality of transponders, each transponder having a random number associated therewith, a counter counting the number of marker signals received from the interrogator and transmission means to transmit an identification code when the number of marker signals received by the transponder equals the random number associated with the transponder.
Another aspect of the present invention provides an RF transponder identification protocol to enable unique identification of one or more transponders by an interrogator operable to transmit a plurality of marker signals, each transponder having a random number associated therewith, a counter to maintain a count of the number of marker signals received from the interrogator, and transmission means to transmit a unique identification code, the protocol comprising the steps of: transmitting marker signals from the interrogator; receiving the marker signals at the respective transponders; each transponder increasing the marker signal count in response to each marker signal received by that transponder; and each transponder having a marker signal count equal to the random number associated with that transponder transmitting a unique identification code.
Additional objects and advantages of this invention will be apparent from the following detailed description of preferred embodiments thereof which proceeds with reference to the accompanying drawings.